1. Field of the Invention
The invention concerns a method for measuring the level of the contents of a container by a capacitive sensor circuit. The invention also concerns a device for performing the method. The container contents in question include liquids and other fluid substances, pourable solids, such as granules, and even gases.
2. Description of the Related Art
It is already known that the capacitive measurement of the level of a liquid in a container can be carried out by submerging two capacitor plates in the container, whose separating gap is filled partly with the liquid and partly with air as a dielectric. The capacitance of the capacitor formed in this way thus consists of the two partial capacitances corresponding to the different dielectric constants of the two media, air and liquid. There is a linear relationship between the total capacitance of this capacitor and the level of the liquid, so that the level can be determined by determining the total capacitance or the displacement current between the two plates.
A device for level measurement is the object, for example, of EP-A 100 564. A disadvantage of this device is that plates or tubes of a precision capacitor must be submerged in the liquid, which is difficult or impossible in many applications. An example of this is the cells of batteries, which would have to be provided with corresponding holes for this purpose. Furthermore, the capacitor would be exposed to corrosion by the electrolytes and to electrical interference by the charge of the batteries. In other applications, for example, the measurement of the contents of fuel tanks in vehicles or aircrafts, there is the danger that the highly combustible liquid could be ignited by the displacement current that is generated, which means that this system cannot be used for such cases.
U.S. Pat. No. 3,119,266 describes a system for level measurement that requires a capacitive sensor circuit, which consists of two capacitor plates, namely, a container wall and an electrode, both of which are in contact with the contents of the container. These two elements form a capacitor, whose changes in capacitance are measured as a function of the changes in the medium between the capacitor plates. The same comments made above in connection with EP-A 100 560 and the same disadvantages thus apply to this system.